The present invention relates generally to devices for coupling a source to a load and more particularly to constant impedance coupling devices for providing impedance matching over a wide band of frequencies between a load having substantially constant impedance and a source, such as an antenna, having an impedance that varies substantially with frequency.
Coupling a broad band antenna to a load presents problems of impedance matching because the impedance of the antenna varies substantially with frequency. In order to minimize the standing wave ratio in the transmission line, the impedance of the antenna must be matched to the impedance of the load. One way of approaching this problem is to design the antenna itself to have the desirable impedance characteristic. This solution is unsatisfactory since it results in a heavy, cumbersome antenna which still does not provide the desired impedance characteristics over a wide enough range of frequencies. Also, the design of an antenna for an impedance characteristic often interferes with the design of the antenna for its primary function.
Another approach to the problem is to provide an inductor-capacitor network in the transmission line and adjust the values of the elements to achieve an impedance match. This solution requires the use of a large number of expensive components, requires adjustment of component values for different frequencies, and is impractical for true broadband applications.
A few electronic circuits have been developed to deal with this problem but none has provided the ideal solution.